As shown by the block diagram of FIG. 1, dithered analog to digital conversion involves adding a random noise signal ("dither") to an input analog signal, converting the combined signal into digital form, and then subtracting the dither signal from the digitized output. This technique improves the resolution and linearity of the conversion by effectively smoothing the quantization errors of the ADC's transfer function.
A problem with dithered ADCs occurs when subtraction of the digital dither signal from the digital output signal fails to completely remove the dither signal. Any remaining residue, or artifact, of the dither signal in the digital output signal results in the digital output signal being an inaccurate representation of the analog input signal.
Dither residue is particularly a problem in situations in which the dither signal is large relative to the input signal (so-called "large-scale" dithered ADCs). In such situations, even slight imperfections or gain variations in the analog portion of the ADC, the dither DAC, or the associated linear amplifiers, can result in imperfect cancellation of the dither in the output signal by subtraction of the original dither signal.
In U.S. Pat. No. 5,187,481 (incorporated herein by reference) there is described a dithered ADC having an active gain control loop to eliminate such gain errors. A representative block diagram is shown in FIG. 2. In that system, the dither signal is correlated with the digital output after subtraction, and the correlated value is accumulated. The correlation indicates the magnitude of the imperfectly cancelled dither residue and is used in a feedback loop to modify the amplitude or gain of the original dither signal. This feedback loop thus assures the subtraction is performed more precisely.
In theory, the foregoing technique is simple. In practice, however, the technique encounters a number of technical complications. The present invention is directed to solutions to these complications so that the full advantages of the foregoing technique can be attained.
One complication is that the loop bandwidth of the feedback loop is very low. This is a necessary result from control theory: the design parameters for minimizing the dither subtraction errors necessitates larger gains in the feedback loop, which in turn yields longer time constants. The consequence of this complication is that at power-on or other initialization, the long time constant of the feedback loop results in a lengthy settling period during which the dither subtraction gradually gets better. If the circuit is employed in a measurement instrument, the instrument's frequency domain noise floor changes as the subtraction errors slowly decrease. Optimum measurements cannot be performed until the noise errors have decreased and the noise floor has settled out.
A second complication relates to overloads. Overloads are common occurrences when signals exceed the maximum limits or ranges of the components--such as the ADC. When an overload occurs, false data values are propagated through the system. In the case of the correlator/accumulator, these false values are also correlated to the dither and therefore corrupt the contents of the accumulator. This results in a shift in the gain of the feedback control signal. Again, if the circuit is employed in a measurement instrument, such an overload manifests itself as a degradation of the instrument's frequency domain noise floor. If the overload is sustained or recurs, the accumulated errors compound. Due to the large time constants of the feedback loop, these errors take a long time--often several minutes--to decrease and settle out after the overload is removed.
In accordance with one aspect of the present invention, the former problem (slow error settling) is alleviated by initializing the accumulator with a nonzero value. The value is chosen to approximate the ultimate accumuland, and serves to set the dither gain to a preestablished value, thereby providing more nearly optimum cancellation of the dither signal immediately on power-up.
Surprisingly, the latter problem (overload) is alleviated in the same manner (initialization of the accumulator), as detailed more fully below.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.